1. Field of the Invention
The present invention is directed to methods and devices for monitoring the acceleration of objects.
2. Discussion of Related Art
Piezoelectric materials are commonly used in transducer and actuator applications. A piezoelectric material generates an electric field in response to applied mechanical force, and generates mechanical force in response to an applied electric field. Transducer applications take advantage of the former of these properties, and actuator applications take advantage of the latter. Examples of piezoelectric materials include quartz crystal (which is a naturally occurring crystal commonly used in oscillators), and certain polycrystaline ceramics, e.g., barium titanate, lead metaniobate, lead [Pb] zirconate titanate (PZT), and the like. These types of ceramics are commonly referred to as piezoceramics.
Piezoceramic elements for use as actuators or transducers may be fabricated by precasting and firing a quantity of piezoceramic material into a desired shape, e.g., a rectangle or circle. After being formed, each element is typically subjected to a treatment called prepolarization during which the dipoles of the element are aligned in a chosen direction. This polarization of the element""s dipoles causes the element to exhibit its piezoelectric properties. One way to prepolarize a piezoceramic element is to attach a pair of electrodes to opposing surfaces of the element, and to apply a strong electric field across the electrodes while keeping the element at a temperature just below its Curie point. When a piezoceramic element is prepolarized in this manner, the element experiences a permanent increase in dimension in the direction of the applied electric field, i.e., between the electrodes, and experiences a permanent decrease in dimension perpendicular to the direction of the applied electric field, i.e., parallel to the surfaces on which the electrodes are disposed.
After a piezoceramic element has been prepolarized, when a dc voltage of the same polarity as the prepolarizing voltage (but of a lesser magnitude) is applied between the element""s electrodes, the element experiences further expansion in the direction of the applied voltage and further contraction perpendicular to the direction of the applied voltage. Conversely, when a dc voltage of the opposite polarity (but of a lesser magnitude) as the prepolarizing voltage is applied between the element""s electrodes, the element experiences contraction in the direction of the applied voltage and expansion perpendicular to the direction of the applied voltage. In either case, the piezoceramic element returns to its original shape after the dc voltage is removed from the electrodes. Therefore, such a piezoceramic element can be used as an actuator insofar as the voltage applied across the element""s plates cause the element""s physical shape to undergo corresponding changes.
This phenomenon also works in reverse. That is, after a piezoceramic element has been prepolarized, when a tension force is applied to the element in a direction parallel to the prepolarization field and/or a compression force is applied to the element perpendicular to the direction of the prepolarization field, the element is caused to expand in the perpendicular direction and contract in the parallel direction. This expansion and contraction, in turn, causes a voltage of the same polarity as the prepolarizing voltage (but of a lesser magnitude) to appear between the electrodes. Conversely, when a compression force is applied to the element in a direction parallel to the prepolarization field and/or a tension force is applied to the element perpendicular to the direction of the prepolarization field, the element is caused to contract in the parallel direction and expand in the perpendicular direction. This contraction and expansion, in turn, causes a voltage of the opposite polarity (but of a lesser magnitude) as the prepolarizing voltage to appear between the electrodes. Therefore, such a piezoceramic element can be used as a transducer insofar as the physical forces applied to the piezoceramic element cause corresponding voltages to be generated between the electrodes.
An example of a prior art acceleration-sensing device 100 which employs a pair of piezoceramic elements as a transducer is shown in FIG. 1. Such a device is disclosed in U.S. Pat. No. 5,631,421, which is hereby incorporated herein by reference. As shown in FIG. 1, the acceleration-sensing device 100 includes a pair of support members 102a and 102b, a piezoceramic beam 104, and a pair of electrodes 106a and 106b. The piezoceramic beam 104 includes two distinct piezoceramic portions 104a and 104b, with a bottom surface 114 of the portion 104a being mated with a top surface 116 of the portion 104b. The beam 104 is sandwiched between the pair of support members 102a and 102b, and the electrodes 106a and 106b are attached, respectively, to a top surface 112 of the portion 104a and a bottom surface 118 of the portion 104b. Each of the portions 104a and 104b is polarized vertically in a direction perpendicular to the top and bottom surfaces of the portions 104a and 104b, but the two portions 104a and 104b are polarized in opposite directions.
In the device 100, a center portion 108 of the beam 104 is held stationary by the support members 102a and 102b, and ends 110a and 110b of the beam 104 are permitted to move freely in response to acceleration of the support members 102a and 102b. The beam 104 is therefore caused to flex when an object (not shown) to which the support members 102a and 102b are attached is subjected to acceleration. When the ends 110a and 110b of the beam 104 flex upward in such a situation, the portion 104a of the beam 104 is subjected to compression forces and is caused to contract (i.e., shorten), and the portion 104b is subjected to tension forces and is caused to expand (i.e., lengthen). Because the portions 104a and 104b are polarized in opposite directions, however, the voltage generated (in response to these compression and tension forces) between the top and bottom surfaces of the respective portions is of the same polarity. Therefore, the voltage produced between the electrodes 106a and 106b when the ends 110a and 110b of the beam 104 flexes upward is equal to a sum of the voltages generated between the top and bottom surfaces of the respective portions 104a and 104b. 
Conversely, when the ends 110a and 110b of the beam 104 flex downward, the top portion 104a of the beam 104 is subjected to tension forces and is caused to expand, and the bottom portion 104b is subjected to compression forces and is caused to contract. Therefore, because the portions 104a and 104b are polarized in opposite directions, the voltage produced between the electrodes 106a and 106b when the ends 110a and 110b of the beam 104 flex downward is also equal to a sum of the voltages generated between the top and bottom surfaces of the respective portions 104a and 104b, but is of an opposite polarity as the voltage produced when the ends 110a and 110b flex upward.
Thus, because the beam 104 flexes in proportion to the acceleration of the object (not shown) to which the support members 102a and 102b are attached, the signal generated between the electrodes 106a and 106b (as a result of the portions 104a and 104b of the piezoceramic beam 104 expanding and contracting when the beam 104 flexes) is indicative of the acceleration of the object.
FIG. 2 is a diagram showing another example of a prior art acceleration-sensing device 200 which employs a pair of piezoceramic elements as a transducer. The device of FIG. 2 is disclosed in U.S. Pat. No. 5,063,782, which is hereby incorporated herein by reference. As shown in FIG. 2, the acceleration-sensing device 200 includes an annular, electrically-conductive support member 202; a pair of circular piezoceramic elements 204a and 204b; a circular conductor 212; and a pair of electrodes 206a and 206b. The support member 202 supports an inner section 208 of each of the piezoceramic elements 204a and 204b and the circular conductor 212, such that an outer perimeter 210 of these components is permitted to flex upward and downward with respect to the inner section 208 (forming a spherical shape) when an object (not shown) to which the support member 202 is attached is accelerated.
The electrode 206a is electrically connected to both a top surface 214 of the piezoceramic element 204a and a bottom surface 220 of the piezoceramic element 204b via the electrically conductive support member 202, and the electrode 204b is electrically connected to both a bottom surface 216 of the piezoceramic element 204a and a top surface 218 of the piezoceramic element 204b via the circular conductor 212. An annular insulating ring 222 is positioned between the support member 202 and the circular conductor 212 to electrically isolate each from the other.
As with the acceleration-sensing device 100 of FIG. 1, each of the piezoceramic elements 204a and 204b of the acceleration sensing device 200 is polarized in a direction perpendicular to its top and bottom surfaces so that each element generates a respective voltage between its top and bottom surfaces when the outer perimeter 210 of the device 200 is flexed. In contrast to the device 100 of FIG. 1, however, the elements 204a and 204b are polarized in the same direction. Therefore, when the outer perimeter 210 flexes upward or downward with respect to the support member 202, the expansion and contraction, or vice versa, of the respective top and bottom piezoceramic elements 204a and 204b causes voltages of the same polarity to appear (in parallel) between the electrodes 206a and 204b (via the circular conductor 212 and the support member 202). This configuration is disclosed as being advantageous because the piezoelectric effects of the two piezoceramic elements 204a and 204b are caused to cancel one another.
Although acceleration sensors such as those shown in FIGS. 1 and 2 function satisfactorily for their intended purposes, they tend to be relatively difficult and expensive to produce, and can easily become damaged. That is, because a relatively large quantity of piezoceramic material is required to produce the piezoceramic components of these devices, the cost of the piezoceramic material itself tends to make these types of acceleration sensors prohibitively expensive to use for many applications. Additionally, the process of producing and properly polarizing the relatively large piezoceramic elements required by such devices can be quite difficult and expensive. Further, because the shapes of the piezoceramic components used in these prior art acceleration sensors are required to be large in order to generate an appreciable voltage, these components tend to be fragile and can become damaged easily.
What is needed, therefore, is an improved method for measuring acceleration.
According to one aspect of the present invention, a method for monitoring acceleration of an object involves providing an apparatus including a non-conductive structure that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure. The apparatus is mounted to the object, and the acceleration of the object is monitored based upon the signal.
According to another aspect of the invention, a method for monitoring acceleration of an object involves providing an apparatus including a structure that flexes in response to acceleration of the object, and a first transducer supported by the structure so as to generate a signal responsive to flexing of the structure, wherein the structure and the first transducer are constructed and arranged such that a neutral axis passes through the structure when the structure flexes, and such that the neutral axis would still pass through the structure when the structure flexes if the first transducer was removed from the structure. The apparatus is mounted to the object, and the acceleration of the object is monitored based upon the signal.
According to another aspect of the invention, a method for monitoring acceleration of an object involves providing an apparatus including a structure that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure, the at least one, transducer including first and second electrodes, each of the first and second electrodes being connected to a respective pair of conductors, the pair of conductors connected to the first electrode being alternately interdigitated with the pair of conductors connected to the second electrode, the apparatus further including a piezoelectric material disposed between adjacent ones of the conductors. The apparatus is mounted to the object, and the acceleration of the object is monitored based upon the signal.
According to another aspect of the invention, a method for monitoring acceleration of an object involves providing an apparatus including a structure, having a surface area, that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure, the at least one transducer covering less than one fourth of the surface area of the structure. The apparatus is mounted to the object, and the acceleration of the object is monitored based upon the signal.
According to another aspect of the invention, a method for monitoring acceleration of an object involves providing an apparatus including a structure that flexes in response to acceleration of the object, and at least one transducer having first and second ends and a middle portion therebetween, wherein the first and second ends of the at least one transducer are mechanically coupled to respective first and second locations on a surface of the structure, without the middle portion being mechanically coupled to the structure, so that the at least one transducer generates a signal responsive changes in a distance between the first and second locations that occur as the structure flexes. The apparatus is mounted to the object, and the acceleration of the object is monitored based upon the signal.
According to another aspect of the invention, a method for monitoring acceleration of an object involves providing an apparatus including a non-circular structure that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure. The apparatus is mounted to the object, and the acceleration of the object is monitored based upon the signal.
According to another aspect of the invention, a method for monitoring acceleration of an object involves providing an apparatus including a structure that flexes in response to acceleration of the object, and at least one capacitor having first and second plates, the at least one capacitor being configured and arranged on the structure to generate a signal between the first and second plates responsive to flexing of the structure. The apparatus is mounted to the object, and the acceleration of the object is monitored based upon the signal.
According to another aspect of the invention, a device for monitoring acceleration of an object includes an apparatus including a non-conductive structure that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure. The device further includes a controller, coupled to the apparatus to receive the signal therefrom, that monitors the acceleration of the object based upon the signal.
According to another aspect of the invention, a device for monitoring acceleration of an object includes an apparatus including a structure that flexes in response to acceleration of the object, and a first transducer supported by the structure so as to generate a signal responsive to flexing of the structure, wherein the structure and the first transducer are constructed and arranged such that a neutral axis passes through the structure when the structure flexes, and such that the neutral axis would still pass through the structure when the structure flexes if the first transducer was removed from the structure. The device further includes a controller, coupled to the apparatus to receive the signal therefrom, that monitors the acceleration of the object based upon the signal.
According to another aspect of the invention, a device for monitoring acceleration of an object includes an apparatus including a structure that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure, the at least one transducer including first and second electrodes, each of the first and second electrodes being connected to a respective pair of conductors, the pair of conductors connected to the first electrode being alternately interdigitated with the pair of conductors connected to the second electrode, the apparatus further including a piezoelectric material disposed between adjacent ones of the conductors. The device further includes a controller, coupled to the apparatus to receive the signal therefrom, that monitors the acceleration of the object based upon the signal.
According to another aspect of the invention, a device for monitoring acceleration of an object includes an apparatus including a structure, having a surface area, that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure, the at least one transducer covering less than one fourth of the surface area of the structure. The device further includes a controller, coupled to the apparatus to receive the signal therefrom, that monitors the acceleration of the object based upon the signal.
According to another aspect of the invention, a device for monitoring acceleration of an object includes an apparatus including a structure that flexes in response to acceleration of the object, and at least one transducer having first and second ends and a middle portion therebetween, wherein the first and second ends of the at least one transducer are mechanically coupled to respective first and second locations on a surface of the structure, without the middle portion being mechanically coupled to the structure, so that the at least one transducer generates a signal responsive changes in a distance between the first and second locations that occur as the structure flexes. The device further includes a controller, coupled to the apparatus to receive the signal therefrom, that monitors the acceleration of the object based upon the signal.
According to another aspect of the invention, a device for monitoring acceleration of an object includes an apparatus including a non-circular structure that flexes in response to acceleration of the object, and at least one transducer supported by the structure so as to generate a signal responsive to flexing of the structure. The device further includes a controller, coupled to the apparatus to receive the signal therefrom, that monitors the acceleration of the object based upon the signal.
According to another aspect of the invention, a device for monitoring acceleration of an object includes an apparatus including a structure that flexes in response to acceleration of the object, and at least one capacitor having first and second plates, the at least one capacitor being configured and arranged on the structure to generate a signal between the first and second plates responsive to flexing of the structure. The device further includes a controller, coupled to the apparatus to receive the signal therefrom, that monitors the acceleration of the object based upon the signal.